ESEEM and EXAFS of oxyCo [tetraphenylporphyrin (TPP)] [1-methylimidazole (1-MeIm) were carried out in order to examine the correlation of oxygen affinity withelectron-nuclear coupling parameters and metal-ligand bond lengths. ESEEM demonstrates that the magnitude of the electron-nuclear hyperfine and nuclear quadrupole couplings to the directly-coordinated 14N of 1-MeIm decrease s the solvent composition is varied from 0 to 50% (v/v) dichloromethane in toluene. For oxyCo[(o-R)1)TPP][MeIm] (where R = -H, -NHCOC(CH3)3, -NHCOCH3, or -NHCONHC6H5 an ortho substituent on one of the four meso phenyls of TPP), couplings to the axial nitrogen decrease with increased electron-withdrawing strength of R, i.e, with increased acidity of the amide proton of R that may interact with the bound dioxygen. EXAFS measurements, and analysis using ab initio EXAFS codes and global mapping, find that the cobalt-axial nitrogen (Nax) bond of oxyCoTPP-1-MeIm shortens by 0.18 q 0.06 when the solvent is changed from 100% toluene (Co-Nax = 2.12 ) to 50% toluene/50% dichloromethane (Co-Nax = 1.94 ). Increasing the polarity of the solvent and of the vicinity of the bound dioxygen increases oxygen affinity of the metal due to an increase in the ionicity of the cobalt-dioxygen bond that is manifested in reduction in electron-nuclear couplings to the axial nitrogen and shortening of the cobalt-axial nitrogen bond. These ESEEM and EXAFS characterizations of metal-ligand interactions demonstrate the correlation of electron-nuclear coupling and metal-ligand bond lengths